As is known, a gas turbine is a machine consisting of a compressor and a turbine with one or more stages, in which these components are interconnected by a rotating shaft and in which a combustion chamber is provided between the compressor and the turbine.
Air from the external environment is supplied to the compressor where it is pressurized.
The pressurized air passes through a duct, terminating in a converging portion, into which a set of injectors supplies fuel which is mixed with the air to form a fuel-air mix for combustion.
The fuel required for the combustion is therefore introduced into the combustion chamber through one or more injectors, supplied from a pressurized network, the combustion process being designed to cause an increase in the temperature and enthalpy of the gas.
A parallel fuel supply system, for generating a pilot flame in the proximity of the mixing duct, is also generally provided in order to improve the stability characteristics of the flame.
Finally, the gas at high temperature and high pressure passes through suitable ducts to reach the various stages of the turbine, which converts the enthalpy of the gas into mechanical energy which is available to a user.
It is well known that the primary considerations in the design of combustion chambers for gas turbines are the flame stability and the control of excess air, the aim being to establish ideal conditions for the combustion.
A second element influencing the design of combustion chambers of gas turbines is the tendency to make the combustion take place as near as possible to the dome of the combustion chamber.
More specifically, the prior art provides for the use of a flame tube or “liner” within the combustion chamber; this has two principal functions.
In the first place, the flame is contained within the tube, thus preventing contact with the outer walls of the combustion chamber, in order to avoid overheating.
Secondly, the tube decelerates and diffuses the flow of the combustion products, preventing the extinguishing of the flame.
Additionally, combustion chambers very commonly have premixing chambers upstream from them, in which air which has previously been used to cool the walls of the combustion chamber is mixed with the fuel.
It is convenient to form a cavity around the flame tube.
This cavity carries pressurized air which circulates in the opposite direction to the flow of combustion products leaving the combustion chamber.
As stated above, this air is used as the combustion air to be mixed with the fuel in the premixing chamber and as the cooling air for cooling both the combustion chamber and the combustion products.
In order to achieve low polluting emissions of nitrogen oxides at all levels of loading of the turbine, the combustion air passes from the cavity, outside the tube flame, to the premixing chamber through apertures in the outer surface of the latter, and can be constricted.
The constriction is applied as a function of the quantity of fuel used, in such a way that the ratio between combustion air and fuel is kept constant at the optimal value.
In the prior art, the flame tube is positioned at the outlet of a truncated conical end connected to the premixing chamber, in the actual combustion region, or the main flame region, of the chamber.
Cooling air, pressurized for example by an axial compressor and circulating in the opposite direction to the flow of combustion products leaving the combustion chamber, flows between the flame tube and the outer walls of the combustion chamber.
The flame tube is connected by means of a truncated conical end to the premixing chamber, and has a cylindrical structure, which essentially contains two distinct regions.
A first region, located around the main flame, comprises a cylindrical casing with no apertures, while the second, longer, region has a set of apertures or holes and channels for guiding the air passing through them in a direction parallel to the wall of the said region.
Additionally, a cavity, whose outer surface has numerous small holes for the admission of air, is created around the truncated conical end.
Thus the pressurized air which passes through these holes creates a large number of air draughts directed towards the outer surface of the first region, thus providing cooling essentially by convection.
In the first region, there are no apertures; this prevents the incoming air from causing incomplete combustion which would give rise to problems of polluting emissions.
In the second region, however, the effect of the cooling air on the completeness of the combustion is less significant, and therefore the wall has numerous apertures, producing a flow of air which passes over the interior of the wall and thus cools it.